Typical building fire alarm systems include a number of fire detectors positioned throughout a building. Signals from those detectors are monitored by a system controller, which, upon sensing an alarm condition, sounds audible alarms throughout the building. Flashing light strobes may also be positioned throughout the building to provide a visual alarm indication. In a system sold by Simplex Time Recorder Company disclosed in U.S. Pat. No. 4,796,025 (hereinafter the '025 patent), the contents of which are incorporated herein by reference, fire detectors are monitored and controlled through common power lines which also serve as communication links. Communications are by means of encoded pulses of voltage drops in power. Notification appliance circuits (NACs) may also be coupled to those communication links through a NAC controller but additional power is applied to the NACs due to the higher power requirements of NACs.
A number of notification appliances comprising audible alarms and strobes generally referred to as notification devices, are typically connected across common power lines on a notification circuit, either directly coupled to the central panel or through a NAC controller on a detector circuit. A first polarity DC voltage may be applied across the notification circuit in a supervisory mode of operation. In this supervisory mode, rectifiers at the notification appliances are reverse biased so that the alarms are not energized, but current flows through the power lines at the notification circuit to an end-of-line resistor and back, allowing the condition of those lines to be monitored. With an alarm condition, the polarity of the voltage applied across the power lines is reversed to energize all notification appliances on the notification circuit.
U.S. Pat. No. 5,559,492 issued to Stewart el al. (hereinafter the '492 Stewart patent), the contents of which are incorporated herein by reference, further discloses that the visual alarms, or strobes, may be synchronized to fire simultaneously with power interruptions, also referred to as synchronization pulses, in the power lines. Additional timing lines for synchronizing the strobes are not required because the synchronizing signals are applied through the existing common power lines.